Somatom Definition tackles coronary need for speed

Siemens' unveiling of the Somatom Definition Nov. 17 in New York City promises to radically change CT scanning.

Technically, the scanner, which packs two imaging chains in a single unit, generates 128 slices per rotation. But Siemens is not marketing that fact. Instead the company is focusing on the speed of the scanner and how its use might change the clinical application of CT. This is no accident.

"In developing this system, we looked at what it would take to redefine the role of CT and to define the future of CT," said Bernd Ohnesorge, Siemens' vice president of CT.

Siemens' strategists decided to break with seven years of slice wars, mounting two detectors at a 90° angle, each containing 32 elements and aligned with its own Straton x-ray tube, to create a system with extraordinary speed. The Somatom Definition generates a slice in 83 msec, half the time of a conventional 64-slice scanner and fast enough to freeze a beating heart, according to the company.

Intuitively, firing two x-ray tubes simultaneously could be expected to create an intolerable increase in dose. Because scan time is minimized, however, the Somatom Definition actually cuts the dose. When applied to the heart, it can acquire as much data in a beat or two as in the five or more required by current scanners. Consequently, x-ray exposure to the patient is expected to drop by half at the same time that power doubles, according to Ohnesorge.

"We can get incredible scan speed, because the Somatom Definition can utilize power reserves up to 160 kW, driven by two 80-kW tubes and generators," he said. "We can utilize these power reserves to scan always at top speed and get high image quality independent of patient size and weight. So even with obese patients, we can get maximum quality and speed."

This power will not come cheap. Customers will pay between $2.3 million and $2.5 million for a Somatom Definition, about a half-million dollars more than any of the current generation 64-slice scanners. Its arrival has been a long time coming, according to Ohnesorge.

The scanner began taking shape two years ago with development of the Straton x-ray tube (DI SCAN 10/29/03), a compact, highly efficient tube compatible with the company's z-Sharp technology, the backbone of the Sensation 64. This platform, launched in 2004, delivers improved spatial resolution through the integration of these two technologies, creating simultaneously two data sets for each point along the axial rotation of the imaging chain (DI SCAN 1/8/04). Combining two such imaging chains doubles temporal resolution and cuts acquisition time in half, as the Somatom Definition demonstrates.

The introduction of Siemens' new scanner represents a leap of faith for the company, as it comes barely a year after the launch of the Sensation 64. Radiology and cardiology are still coming to grips with this new generation of 64-slice scanners. The Somatom Definition addresses the questions already raised with a different technology, which may lead to debate over which technology is best.

To Ohnesorge, the question is academic. The speed of the system will eliminate motion artifact when imaging the heart, making it the logical choice for cardiology applications, especially coronary CTA, he said.

Beta blockers are often used in cardiac CT to slow the heart and, thereby, reduce motion artifact. This use is far from ideal, and physicians worry that it may obscure anomalies.

"We feel that with this scanner, there will be no need to use beta blockers to prepare patients," Ohnesorge said.

Physicians have similar reservations about the use of algorithms that combine data from multiple slices taken at the same point in subsequent heartbeats. Combining and averaging these data to get a high-quality image may miss subtle abnormalities. This process also exposes patients to a relatively high dose of x-rays, because the heart must be scanned for several seconds.

"With conventional single-source scanners, the temporal resolution is limited such that the temporal window is quite long," Ohnesorge said. "This is not required any more because with the Somatom Definition, you can image every beating heart without motion artifact at a reduced temporal window where you apply radiation for a very short time interval."

The single-source scanner acquires its slice after a 180° rotation. With two imaging chains, mounted at a right angle to one another, the same information can be acquired from those two sources with only a 90° turn.

The short scan time on the Somatom Definition is achieved without the use of segmentation algorithms that compile data from sequential rotations. If Siemens were to use such algorithms, temporal resolution would double again to about 42 msec. Ohnesorge does not consider such mathematical manipulations necessary, however.

"It has been shown in EBT (electron-beam tomography) that if you are able to go below 100 msec, there is no motion artifact in coronary artery imaging," he said.

Aiding the adoption of the Somatom Definition are software developments such as syngo Circulation, which automatically tracks coronary vessels and performs cardiac functional analyses. Automatic vessel tracking is also integrated into syngo InSpace, which supports other CTA exams.

The Somatom Definition may also be valuable in acute care, according to Ohnesorge. It might be able to handle a broad range of applications from trauma, including stroke, to the assessment of patients with chest or abdominal pain. It might also be applied in virtual colonography and pulmonary imaging.

Ohnesorge defers to luminaries, however, to determine when and how the system should be used. Answers are on the way. An evaluation site for the Somatom Definition is operating at the University of Erlangen in Germany. Ten more such sites are scheduled for installation by spring 2006.

Research will be particularly important in gauging the clinical value of the Somatom Definition's ability to acquire data simultaneously at two different energies.

"Dual-energy scanning will lead to the potential to have two data sets that you can add and subtract to get new information," Ohnesorge said.

This technique might be used in cardiovascular imaging to differentiate the vessel wall from surrounding tissue. It might be used to isolate bone and blood vessels, as in the case of complex vasculature near the skull or across the knee. It might also be used in the lung to characterize pulmonary arteries and nodules or in the liver to differentiate fatty lesions from carcinoma. But these capabilities remain largely unexplored and undocumented.

"There will be lots of potential for doing research to look at the kV settings, at combining these settings with the right dose protocols and mA applications. We might have to adjust how we use contrast agents to optimize the image quality," he said.

Determining the potential of this dual-detector technology and optimizing its application will take years. But system sales will not have to wait. Ohnesorge expect the company to sell as many as 200 Somatom Definitions by the end of 2006. One hundred of these might be installed during this time. If so, the scanner next year will contribute recognized gross revenues of between $230 million and $250 million, an unprecedented amount for any single CT product.